Host and parasite populations live in highly variable natural environments. This thesis explores how the population dynamics and genetic population structure of an important spring-bloom phytoplankton species, the diatom Asterionella formosa, are affected by such environmental variation and by parasitism by its fungal parasite, the chytrid Zygorhizidium planktonicum. Asterionella is ... read more often the dominant spring-bloom species in temperate lakes and rivers. The spring-bloom is a key event in the yearly seasonal dynamics of lakes and provides the first peak in primary production determining all further events in the lake foodweb. Even though Asterionella reproduces predominantly by asexual cell division, it shows high genetic diversity (i.e. high percentage of different genotypes) in natural populations in Lake Maarsseveen (NL). This is surprising as we would expect selection for the best adapted genotypes and therefore erosion of genetic diversity at population level. How genetic diversity is maintained in populations of asexually reproducing organisms is still an actively debated question in ecology. Two mechanisms, amongst others, may promote maintenance of such high population genetic diversity: (i) fluctuating selective advantage of genotypes in variable environments (i.e. genotype-by-environment interactions) and (ii) parasite mediated selection against (initially) common host genotypes. Both theories can be combined in the disease triangle concept proposed by McNew and Stevens in 1960. This concept explains the occurrence and severity of disease as an interaction and interdependency of three factors: the host, the parasite and their shared environment. In this thesis we combined field studies in a natural host-parasite population in Lake Maarsseveen with controlled laboratory experiments. We showed that, over the last 30 years, climate change has changed the parasite occurrence pattern. Experimental and field studies showed that the host has a cold and a hot thermal refuge from chytrid infection, so-called windows of infection-free growth. If winter water temperatures stay above this threshold, the parasite remains active, hampers the development of a host bloom and thereby removes its own basis for an epidemic development later in the season. Temperature is, however, not the only environmental factor that affects the population dynamics of hosts and parasites. In stratified lakes, also other abiotic environmental conditions such as light intensity and nutrient availability change with season but also with depth. Host and parasite populations responded to different environmental factors, and the importance of environmental factors changed again with depth. Moreover, high levels of parasite infection affected the genetic architecture of its host population. Laboratory experiments testing the role of temperature, phosphorus concentration and / or parasitism showed that the selective advantage and the susceptibility to disea se of different Asterionella genotypes depended on the level of such environmental factors. That means that the fitness and susceptibility to disease of a particular Asterionella genotype changed with temperature environment. Hence, environmental variability in time and space may change the potential selection pressure on hosts as well as the specificity of host-parasite interactions. Such context dependence in genotype performance can support the maintenance of genetic diversity even in an isolated population of asexually reproducing organisms show less